Safety instrumented system (SIS) function

ISA 5.1-2009/2022 (general instrument and function symbols) / IEC 61511

Drawn as

A diamond inside a square (with a bar when operator-accessible), carrying a trip function such as LSHH or PSHH. Many legends add "SIS" or a distinct colour or letter.

Meaning

A trip or permissive in the safety instrumented system: PSD/ESD logic on independent SIL-rated hardware. Not physical. Each one implies dedicated field sensors (often LZT/PZT, or a second transmitter) and final elements (ESDV, BDV, motor trips).

Variants

ISA 5.1-2009 introduced the diamond-in-square option for SIS. ISA describes the 2022 edition as a corrections release, and no change to this symbol was reported (the 2022 text was not checked)
Many operators mark SIS loops with Z (e.g. PZT, LZT, XZV) or a "S" prefix; this is company practice, not ISA
Some legends use a hexagon or a double-bordered diamond for the SIS and keep diamond-in-square for PLCs
A note such as CMS beside it puts the trip in the machinery protection / condition monitoring system (API 670 rack), not the process SIS: see cms-system